section 4: synoptic easterly waves
DESCRIPTION
Section 4: Synoptic Easterly Waves. Section 4: Synoptic Easterly Waves. 4.1 Introduction 4.2 The Mean State over West Africa 4.3 Observations of African Easterly Waves 4.4 Theory 4.5 Modeling 4.6 Hot Topics: 4.6.1 Genesis 4.6.2 Scale Interactions - PowerPoint PPT PresentationTRANSCRIPT
Section 4: Synoptic Easterly Waves
Section 4: Synoptic Easterly Waves
4.1 Introduction
4.2 The Mean State over West Africa
4.3 Observations of African Easterly Waves
4.4 Theory
4.5 Modeling
4.6 Hot Topics:
4.6.1 Genesis
4.6.2 Scale Interactions
4.6.3 Relationship to Tropical Cyclogenesis
4.7 Easterly Waves in other Tropical Regions
4.8 Final Comments
4.1 Introduction
• Westward moving synoptic waves characterize the whole tropics
• They are tropospheric waves, that modulate the rainfall and move at about 8m/s and have wavelengths of 2000-4000km.
4.1 Introduction
• The environments that they are embedded in varies around the tropics, and so details of the wave characteristics also vary.
4.1 Introduction
The emphasis here will be on African Easterly Waves (AEWs)
Have a strong influence on daily rainfall patterns over Africa and tropical Atlantic
Most Atlantic Tropical Cyclones are generated in association with AEWs
AEWs
MCSs
SAL
TC
4.2 The Mean State over West Africa
Burpee, R.W. 1972 The origin and structure of easterly waves in the lower troposphere of North Africa, J. Atmos. Sci. 29, 77-90
Notable Features: 600mb African Easterly Jet (AEJ)
Upper-level Tropical Easterly Jet (TEJ)
Low-level Monsoonal Westerlies
Low-level Easterlies north of the AEJ
Upper-level Westerly Jet to the North
4.2 The Mean State over West AfricaReed, R.J., Norquist, D.C. and Recker, E.E., The structure and properties of African wave disturbances as observed during Phase III of GATE, Mon. Wea. Rev. 105, 317-333 (1977).
PV View of the African Easterly Jet
Discussion
Consider the meridional contrasts in convection (next slide) and the diabatic source/sink term in the PV-equation.
Schematic of African Easterly Jet
θ
50oCθ
θe
90oC
θe
AEJ
20oC60oC
4.2 The Mean State over West Africa
Thorncroft and Blackburn 1999
Mean 700hPa U wind, 16th July – 15th August 2000
Zonal Variations in the Mean State
Berry and Thorncroft 2005
925hPa 315K PV
• Strong baroclinic zone 10o-20oN • PV ‘strip’ present on the cyclonic shear side of AEJ.
925hPa e
• High e strip exists near 15oN
Zonal Variations in the Mean State
4.3 Observations of African Easterly Waves
Carlson, T.N., 1969a: Synoptic histories of three African disturbances that developed into Atlantic hurricanes. Mon. Wea. Rev., 97, 256-276.
Carlson, T.N., 1969b: Some remarks on African disturbances and their progress over the tropical Atlantic. Mon. Wea. Rev., 97, 716-726.
Burpee, R.W., 1970: The origin and structure of easterly waves in the lower troposphere of North Africa, J. Atmos. Sci. 29, 77-90.
Reed, R.J., Norquist, D.C. and Recker, E.E., 1977: The structure and properties of African wave disturbances as observed during Phase III of GATE, Mon. Wea. Rev. 105, 317-333
Thorncroft, C.D. and Hodges: 2001 K.I., African easterly wave variability and its relationship to tropical cyclone activity, J. Clim. 14, 1166-1179 (2001).
Kiladis, G., C. Thorncroft, and N. Hall, 2006: Three-Dimensional Structure and Dynamics of African easterly waves: part I: Observations, J. Atmos. Sci., 63, 2212-2230.
Mekonnen, A., C. Thorncroft, and A. Aiyyer, 2006: On the significance of African easterly waves on convection, J. Climate, 19, 5405-5421.
Berry, G., Thorncroft, C.D. and Hewson, T. 2006 African easterly waves in 2004 – Analysis using objective techniques Mon. Wea. Rev., 133, 752-766
4.3 Observations of African Easterly Waves
Carlson 1969ab
Carried out case studies of several AEWs
Peak amplitudes at 600-700mb and at surface
Eastward tilt with height from the surface to the level of the AEJ
Two cyclonic centers at low-levels
Synoptic variations in cloud cover
Peak of cloudiness close to AEW trough
4.3 Observations of African Easterly Waves
Burpee (1970)
Eastward tilt beneath the AEJ – Westward tilt above the AEJ
Northerlies dry and warm
Southerlies wet and cold
4.3 Observations of African Easterly Waves
Composite AEW structures from phase III of GATE (after Reed et al, 1977). (a) and (b) are relative vorticity at the surface and 700hPa respectively with a contour interval of 10 -5s-1. (c) and (d) show percentage cover by convective cloud and average precipitation rate (mm day-1) respectively. Category 4 is location of 700hPa trough and the “0” latitude is 11oN over land and 12oN over ocean.
Reed et al, 1977
4.3 Observations of African Easterly Waves
Thorncroft and Hodges (2001)
Three Dimensional Structure of Easterly Wave Disturbances over
Africa and the Tropical North Atlantic
George N. Kiladis 1
Chris D. Thorncroft 2
Nick M. J. Hall 3
1 NOAA Aeronomy Laboratory, Boulder, CO2 Dept. of Atmospheric Sciences, SUNY, Albany,
NY3 LTHE, Grenoble, France
Space-Time Spectrum of JJA Antisymmetric OLR, 15S-15N
Wheeler and Kiladis, 1999
Regression Model
Simple Linear Model:
A separate linear relationship between a predictor at a grid point and a parameter at every other grid point is
obtained:
y = ax + b
where: x= predictor (TD-filtered OLR at 10N, 10W)
y= predictand (u or v wind at any grid point)
Maps or cross sections at lag can then be constructed to show the evolution of the dynamical fields versus the
predictor
OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2) at 10N, 10W for June-September 1979-1993
Day 0Streamfunction (contours 1 X 105 m2 s-1)
Wind (vectors, largest around 2 m s-1)OLR (shading starts at +/- 6 W s-2), negative blue
OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2) at 10N, 10W for June-September 1979-1993
Day-4Streamfunction (contours 1 X 105 m2 s-1)
Wind (vectors, largest around 2 m s-1)OLR (shading starts at +/- 6 W s-2), negative blue
OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2) at 10N, 10W for June-September 1979-1993
Day-3Streamfunction (contours 1 X 105 m2 s-1)
Wind (vectors, largest around 2 m s-1)OLR (shading starts at +/- 6 W s-2), negative blue
OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2) at 10N, 10W for June-September 1979-1993
Day-2Streamfunction (contours 1 X 105 m2 s-1)
Wind (vectors, largest around 2 m s-1)OLR (shading starts at +/- 6 W s-2), negative blue
OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2) at 10N, 10W for June-September 1979-1993
Day-1Streamfunction (contours 1 X 105 m2 s-1)
Wind (vectors, largest around 2 m s-1)OLR (shading starts at +/- 6 W s-2), negative blue
OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2) at 10N, 10W for June-September 1979-1993
Day 0Streamfunction (contours 1 X 105 m2 s-1)
Wind (vectors, largest around 2 m s-1)OLR (shading starts at +/- 6 W s-2), negative blue
OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2) at 10N, 10W for June-September 1979-1993
Day+1Streamfunction (contours 1 X 105 m2 s-1)
Wind (vectors, largest around 2 m s-1)OLR (shading starts at +/- 6 W s-2), negative blue
OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2) at 10N, 10W for June-September 1979-1993
Day+2Streamfunction (contours 1 X 105 m2 s-1)
Wind (vectors, largest around 2 m s-1)OLR (shading starts at +/- 6 W s-2), negative blue
OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2) at 10N, 10W for June-September 1979-1993
Day+3Streamfunction (contours 1 X 105 m2 s-1)
Wind (vectors, largest around 2 m s-1)OLR (shading starts at +/- 6 W s-2), negative blue
OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2) at 10N, 10W for June-September 1979-1993
Day+4Streamfunction (contours 1 X 105 m2 s-1)
Wind (vectors, largest around 2 m s-1)OLR (shading starts at +/- 6 W s-2), negative blue
OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2) at 10N, 10W for June-September 1979-1993
Day+5Streamfunction (contours 1 X 105 m2 s-1)
Wind (vectors, largest around 2 m s-1)OLR (shading starts at +/- 6 W s-2), negative blue
4.3 Observations of African Easterly Waves
All the previous slides refer to composite AEW structures
They say little about the significance of AEWs on convection and
They say little about how these structures might be manifested on a weather map or how they may vary in space and time.
The next slides address the significance issue from Mekonnen et al (2006)
This will be followed by some maps of individual AEWs (Berry et al 2007).
Shaded region: power > red noise
Central Africa
E. Africa
Significant time scales:2-6 days & at 1 day.
Peak periods change from west to east
E. Atlantic
W. Africa
TB variance (in K2)
E. Atlantic (5-10N, 40W-20W)Land (10-15N, 15W-40E)
Shaded region: power > red noise
Central Africa
E. Africa
Significant time scales:2-6 days & at 1 day.
Peak periods change from west to east
E. Atlantic
W. Africa
TB variance (in K2)
Shaded region: power > red noise
Central Africa
E. Africa
Significant time scales:2-6 days & at 1 day.
Peak periods change from west to east
E. Atlantic
W. Africa
TB variance (in K2)
Shaded region: power > red noise
Central Africa
E. Africa
Significant time scales:2-6 days & at 1 day.
Peak periods change from west to east
E. Atlantic
W. Africa
TB variance (in K2)
Central Africa
E. Africa
E. Atlantic
W. Africa
TB variance (in K2)
Significant time scales:2-6 days & at 1 day.
Peak periods change from west to east
Shaded region: power > red noise
2-6d TB variance (shaded >140K2)
2-6d contribution:25-35% over land,35-40% over ocean
west-east variance is nearlythe same
Variance explained by 2-6d TB
(shaded > 20%)
Comparison with dynamic measures …..
2-6d 700-hPa variance
2-6d 850-hPa variance
Variance in the west are higher than in the east!
(shaded >5m2s-2)
Land: maximum along 10N, south of the AEJ, near peak convective region.Ocean: near 20N
Land: maximum to the north of AEJ, and over the coast, near peak convective regionOcean: within ITCZ
315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.
Diagnostics for highlighting multi-scale aspects of AEWs
Berry et al 2006
315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.
315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.
315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.
315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.
315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.
315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.
315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.
315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.
315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.
315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.
315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.
315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.
315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.
315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.
315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.
315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.
Summary of the observed AEJ and AEWs
Summary
AEJ: Consists of two prominent PV anomalies; a positive PV anomaly on the cyclonic side of the AEJ that is diabatically generated in the region of peak rainfall and a negative PV anomaly that is diabatically generated in the heat low region.
AEWs: AEWs have significant circulation anomalies at the level of the AEJ and at the surface. They tend to tilt against the horizontal and vertical shear of the AEJ – this tells us something about the growth mechanisms to be discussed in the next section.